Various inventions have improved the contact conditions of a vehicle tread since the invention of the wheel. In recent years, wheeled vehicles are being used more and more in winter so that demand has particularly increased for skid prevention on ice. Conventional means for improving such conditions may be divided broadly into two categories:
1) On the ground; this category consists of two types: One is a permanent type such as heating pavements with a drawback of high cost. Another is a temporary type such as a traction pad, or a so called “snow helper”, which is a ragged thin board, commonly used for emergencies in cold regions of Japan. The drawback is not only their limited length but it is troublesome work to place these on the ground.
2) On vehicles; this category is a type to improve the wheel function on the road, by using treading pieces such as tire chains, studded tires or caterpillars. Although solving the above-mentioned drawbacks with endless motion of treading pieces, this category has drawbacks of damaging the road or the vehicle itself, since many drivers leave them on continuously even when they are unnecessary.
All the drawbacks of the above-mentioned systems can be solved by a removable type of track treading piece used only when necessary. Therefore many inventions have been proposed to realize this, using a device with circular motion of track treading pieces such as the patent document (1). The devices of these inventions, however, have the basic problem of the orbit discrepancy between a straight line of the tread and a cycloid of the track treading pieces. The discrepancy causes vibration, since the track treading pieces are pulled out continuously from underneath the tread. The friction coefficients are lowered and the track treading pieces and other related parts are subject to high stresses. In order to prevent damage, the devices are made stout and heavy. Therefore a limited number of special vehicles could use it. Besides, special techniques and facilities are required to make the installation so that the maintenance is costly and time consuming.
In order to solve problems of the orbit discrepancy, a treading device for wheels of the patent document (2) was devised. A smaller and lighter device was made by introducing a partly linear motion in the orbit of the treading pieces under the tread. The remote control or, on-off operation, from the driver's seat has been tested with a wire, a radio wave, electricity or radiation. The remote control is being developed to make it possible for the driver to control the position of the treading pieces from the driver's seat.
The treading device, however, has not been put to practical use because of the following drawbacks:
1) The large device is subject to serious damage, projecting from the vehicle body. All parts, even expendable fins trodden by the wheel, are fixed firmly at a complicated position behind the wheel. It is time-consuming and costly to repair the treading pieces even in well-equipped factories. The repair work, taking time and space, is not possible on the spot. Ruts in snow are deep and the minimum amount of snow is removed in cold regions. Therefore the device had to be overdesigned with the numerous safety factors just to avoid the repair work, which was similar to other conventional devices. The drawbacks are due to the excessive safety factor, raising the cost and restricting the use to very low speeds. The following interrelated drawbacks prevent practical use:
A) Special facilities are required for installation or removal.
B) Applicable vehicles are limited due to the size of the device,
C) Parts are costly since these are not mass-produced,
D) A powerful power source such as an air compressor is required.
E) Ice chips or stone can be sent flying making driving dangerous.
F) Driving is accompanied by vibrations and a deafening noise.
G) Driving is limited to a very slow speed.
2) Only limited kinds of materials, made of thick rubber sheet, are available for the treading pieces, since durability matters most. Besides, the retainer which holds the treading pieces is fixed behind the wheel. Accordingly it is not possible to choose the appropriate treading pieces in given road conditions in terms of the climate, the geography or uses.
3) Even the carefully designed device can be damaged when the tread falls into a dent such as a deep rut in the frozen road, since the device is subject to the whole weight of the vehicle. Accordingly the driving path has to be limited to avoid damage.
[Patent Citation 1]
    U.S. Pat. No. 4,299,310 “Anti-skid device for motor vehicles 1981”[Patent Citation 2]    U.S. Pat. No. 5,236,067 “Treading device for wheels 1993” (Japanese Patent 2545543 Treading device for wheels)